6.2: The Process of Photosynthesis
- Page ID
- 133650
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At09kg, Wattcle, Nefronus At09kg: original Wattcle: vector graphics Nefronus: redoing the vector graphics, CC BY-SA 4.0, via Wikimedia Commons
Photosynthesis is composed of two main steps -- the light dependent and light independent reactions. The light dependent reaction is where energy from light is used to harvest electrons from water. Light is absorbed by chlorophyll in the form of photons. When chlorophyll absorbs a photon, one of its electrons gets knocked away. A water molecule is then split and one of its electrons replaces the one chlorophyll lost.
These electrons are then passed on to the light independent reaction. In the light independent reaction (also called the Calvin cycle), these electrons are used to “fix” carbon. Carbon dioxide is taken in through the leaves’ stomata and brought into the chloroplast. Then, an enzyme called RuBisCO strips oxygen away and fixes the carbon to another molecule called RuBP. After cycling through the Calvin cycle, a three-carbon molecule called G3P is formed, with RuBP being recycled to be used again later.
Daniel Mayer (mav) - original imageVector version by Yerpo, CC BY-SA 4.0, via Wikimedia Commons
For this exercise, we will observe the consumption of \(\mathrm{CO}_2\) from photosynthesis. Carbon dioxide (\(\mathrm{CO}_2\)) is taken in through the leaves of a plant to be used as a carbon source when building carbohydrates. We will use phenol red to detect the use of \(\mathrm{CO}_2\). Phenol red appears red when in the presence of a neutral and basic pH, but becomes yellow when the pH becomes acidic. \(\mathrm{CO}_2\) becomes carbonic acid when in water, making the liquid slightly acidic. Prior to class, your instructor has added \(\mathrm{CO}_2\) to your phenol red, giving it a yellow color. If \(\mathrm{CO}_2\) is used up, the solution will become less acidic, turning the solution red.
Materials:
- 2 test tubes with a test tube rack
- 1 sprig of Elodea
- Phenol Red (with \(\mathrm{CO}_2\) already added)
Procedure:
- Obtain 1 small branch of Elodea, approximately 2-3 cm long.
- Place 1 branch of Elodea in Tube 1.
- Add enough phenol red (now yellow from the added \(\mathrm{CO}_2\) ) to cover the Elodea in Tube 1.
- Add the same amount of phenol red to Tube 2. (This tube should only have phenol red.)
- Leave the test tubes in a sunny part of the room (such as a window sill) or outside in the sun for 45 minutes.
- After 45 minutes, record the color in the space below.
|
Tube |
Color Before |
Color After |
|---|---|---|
|
With Elodea |
||
|
Without Elodea |